Preheat the oven to 350°F.

Spray a 9x5-inch loaf pan with nonstick cooking spray. Set aside.

In large mixing bowl, use a mixer to cream together the butter, granulated sugar, brown sugar, and vanilla extract on medium-high speed until smooth and combined, 1-2 minutes. Add the eggs and beat for a minute longer, until the mixture is pale and fluffy. Add the sour cream and milk. Continue to beat on medium-high speed, scraping down the sides of the bowl with a rubber spatula as needed, until fully incorporated.

Add the flour, cocoa powder, baking powder, baking soda, and salt to the bowl. Beat the mixture on medium-low speed, scraping down the sides of the bowl with a rubber spatula, until just combined. Do not over-mix.

Fold in the chocolate chips. Pour the batter into the prepared loaf pan.

Bake the bread on middle rack of oven until a toothpick inserted in the center comes out clean, 55-65 minutes.

Check to see that bread is done. Remove from oven or add time as needed.

Allow the bread to rest in the pan for 10 minutes, then release the bread from the pan onto a cooling rack. Allow to cool for at least 1 hour before slicing and serving. Leftover bread may be stored in a large zip-top bag at room temperature for 1 day, in the refrigerator for 4 days, or in the freezer for up to 3 months.